In the field with which our invention is concerned, the use of capacitive measuring systems for measuring film thickness is known in the art. Also, the use of direct-measurement and contact-type measuring systems for measuring film thickness is known in the art.
One form known in the art is disclosed in U.S. Pat. No. 3,764,899, issued Oct. 9, 1973. This patent measures film thickness by passing the film through a capacitance sensor, wherein variations in film thickness are sensed as dielectric thickness variations between the capacitance members, and are detected as variations in a signal which is applied to the capacitance members.
Another form of capacitance measuring device is disclosed in U.S. Pat. No. 4,952,882, issued Aug. 28, 1990, which was submitted as an improvement in the construction of the capacitance sensor disclosed in the foregoing United States patent. This capacitance sensor assembly also measures changes in thickness of a dielectric film, such as plastic film, where the sensor components are constructed from materials having low coefficients of linear temperature expansion, resulting in a claimed measurement device which provides low measurement errors resulting from temperature effects on material expansion and dielectric changes.
The present invention is an improvement in the construction and calibration of the capacitive measuring system and related transport for serially examining plastic film material.
Measurement instruments of the type for which the invention finds utility are typically used to measure plastic film thicknesses in the range of 0.001-0.100 inch. Measurement accuracy, of both the film thickness and of thickness measurement locations, should be maintained over a range of ambient temperatures and relative humidities which are found in the typical environment where such instruments are used. A problem in prior art measurement devices has been the inability to maintain accurate measurements of film thickness and thickness measurement location over a range of ambient temperature and humidity changes. A second problem in prior art measurement devices, since the capacitive measurement technique is an indirect-measurement technique, has been the requirement to recalibrate the device for the many different types of plastic film products manufactured in a typical plant environment. Prior art devices must be recalibrated for each film type, with its different dielectric properties, to provide accurate absolute thickness information.